A digital color reproduction system such as a color fax, or a digital color copier includes a scanner for scanning a color image to produce scanner color signals representing said color image, a digital image processor for processing the scanner color signals to produce printer color signals, and a printer for reproducing the color image.
A color reproduction system is said to be "closed" if the input images to the system and output images from the system are printed on the same media (paper or film) using the same dye set. One example of a closed color reproduction system is a combination of a film scanner which scans color negatives, and a film writer which writes on color negatives. Another example is a system consisting of a reflection scanner which scans thermal color prints, and a thermal color printer which produces duplicate thermal color prints.
In a color reproduction system, the goal is often to make the reproductions match the originals as nearly as possible. In order to do so, a transform is employed in the digital image processor which maps the scanner color signals, normally in RGB space, to the printer color signals, normally in CMY space so that the system faithfully reproduces the original images. Often the transform is implemented by employing a relatively sparse three dimensional lookup table and a process of three dimensional interpolation between values in the table. The process of deriving such a transform is called system color calibration. If the system is closed, the process is called closed system color calibration, which is a special case of system color calibration.
FIG. 1 describes a conventional approach to digital color reproduction system calibration. A color scanner 10 produces scanner color signals RGB, which are processed by a digital image processor 12 into printer color signals CMY for printer 14. The calibration of the system is performed according to the prior art in two major steps. The first step is to derive a color transform f() 16 to convert the scanner color signals such as RGB into color signals in a device independent color space (DICS) such as CIEXYZ, L*a*b*, L*u*v*. The second step is to derive a transform g() 18 to convert color signals from the DICS to printer color signals CMY. The two transforms f() and g() may then be combined into one transform h() which converts scanner color signals into printer color signals directly. The transform h() is commonly implemented by employing a relatively small 3D LUT and a process of tri-linear interpolation between values in the lookup table as depicted in FIG. 2.
Calibrating a color reproduction system in this way involves: (i) generating a set of color patches on the printer; (ii) measuring the color patches using an optical instrument such as a spectroradiometer; (iii) using a mathematical method such as regression to derive the transform g() based on the measured data; (iv) scanning a set of test patterns; (v) measuring the test patterns using an optical instrument such as spectroradiometer; and (vi) employing a mathematical method such as regression to derive the transform f() based on the scanned data. Calibrating a system in such a way is non-trivial and it cannot be done automatically.